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Stable isotope signatures in soil-derived N[O.sub.3.sup.-] provide an opportunity for environmental source tracing, but rapid changes in N transformation rates caused by sampling disturbance may create an artifact in the signature of extracted N[O.sub.3.sup.-]. To study this, we measured net nitrification and ammonification rates and the [[delta].sup.15]N and [[delta].sup.18]O of N[O.sub.3.sup.-] in soils from a watershed in Camels Hump State Forest, Vermont, known to be sensitive to sampling disturbance. Eleven Oa and one A horizon samples (C/N ratio 15-25, pH 3.34.2) had N[O.sub.3.sup.-] extracted for isotope analysis approximately 30 min after sampling and again after a 2- to 3-d incubation period at 10[degrees]C. Net nitrification rates during incubation were rapid and linear (0.9-15.3 [micro]mol [L.sup.-1] soil [h.sup.-1]), with increases in N[O.sub.3.sup.-] detectable within 1 h after sampling. The [[delta].sup.15]N and [[delta].sup.18]O of soil N[O.sub.3.sup.-] changed dramatically between field extraction and extraction after the incubation period. Soils that were initially relatively enriched in [[delta].sup.15]N[O.sub.3.sup.-] became more depleted after incubation, as much as 13.5%o lighter. Soils that were initially relatively depleted in [[delta].sup.15]N[O.sub.3.sup.-] became more enriched after incubation, as much as 16.5%o heavier. The latter had high net nitrification rates and low final N[H.sub.4.sup.+] concentrations, probably causing the enrichment of [[delta].sup.15]N in N[O.sub.3.sup.-] due to a diminishing substrate pool. The [[delta].sup.18]O of soil N[O.sub.3.sup.-] showed little change during incubation and changes were not significantly related to changes in the [sup.15]N of N[O.sub.3.sup.-], suggesting that denitrification was not a primary mechanism. Added enriched [sup.15]N[H.sub.4]Cl (111.9%o) was rapidly incorporated into the soil N[O.sub.3.sup.-] pool in a pattern supportive of a mechanism due to increased nitrification rates. Soil sampling disturbance can dramatically alter the isotopic signature of soil N[O.sub.3.sup.-], and the isotopic signature of extracted N[O.sub.3.sup.-] may not be a reliable environmental tracer. Abbreviations: DI, deionized distilled.

Abstract: The degree of wetness in forest surface soils has an effect on chemical and biological processes but is not easily measured. The high spatial variability in carbon (C) concentration [...]

Trace metal accumulation by Mn in synthetic oxides and soil nodules has been attributed to specific adsorption and oxidation at Mn oxide surfaces, yet little is known about trace metal interactions with Mn in bulk soil. We investigated competitive effects of trace metal pretreatment on Cr oxidation in well-aerated, high-Mn soils, as well as accumulation of added Pb, Co, and Cu by soil Mn using microfocused synchrotron x-ray fluorescence ([micro]SXRF). Short-term equilibrations of divalent Mn, Co, Pb, Cu, and Ni with soil samples weighed to contain equivalent amounts of N[H.sub.2]OHxHCI-extractable Mn resulted in substantial interference in Cr oxidation, confirming metal interactions with Cr-oxidizing sites on Mn oxide surfaces. Interference by Cu and Pb was greatest in samples likely to be low in competing sorbents, that is, low-pH soils and smaller samples of higher Mn soils, respectively. Strongest interference in Cr oxidation resulted from Mn and Co addition, suggesting specific affinity for soil Mn oxide surfaces, and was greatest in high-pH, high-Mn-valence soils previously shown to have the greatest Cr oxidation capacity. Nickel showed the weakest effect. Micro-SXRF scans revealed substantial spatial correlation of soil Mn with added Co and Pb, but Pb microdistribution was equally correlated with soil Fe. Only modest overlap of soil Mn with added Cu was observed. Our data suggest that specific affinity of Pb, Cu, and Ni for soil Mn oxides was weaker than that of Mn and Co. Higher Mn oxide valence may enhance sorption and subsequent oxidation of these oxidizable metals.

Due to their high degree of reactivity, Mn oxides in soil systems may exert a greater influence on trace metal chemistry than that suggested by their relatively low abundance. In particular, Mn is the only known oxidizer of trivalent Cr in soils. We investigated soil properties that influence the Cr oxidizing capacity of Mn oxides in eight well-aerated high Mn soils. Total and easily reducible Mn abundance were quantified by extraction with 1.5 M N[H.sub.2]OH*HCl and 0.02 M hydroquinone. Relative average Mn oxidation state in soil samples was determined by x-ray absorption near edge structure spectroscopy (Mn-XANES) main edge energy position. Soils ranged in percentage of N[H.sub.2]OH*HCl-extractable Mn between 0.14 and 1.27, pH between 4.4 and 7.2, and percentage of C between 9.0 and 27.2. Manganese-XANES spectra showed that most of the study soils had a high Mn(IV)/Mn(III) ratio with edge energy position intermediate to that of a synthetic birnessite and a synthetic pyrolusite. In these high Mn soils, Mn-XANES edge energy was positively correlated with soil pH, suggesting a linear increase, over the normal range of soil pH, in the Mn(IV)/Mn(III) ratio of soil oxides. Soils with more total reducible Mn generally demonstrated greater net Cr(VI) production, but this pattern was moderated by soil pH and relative Mn oxidation state. High Mn soils with low pH and Mn oxidation state were weaker Cr oxidizers than their Mn abundance would suggest. Our data provide evidence that greater Mn abundance and greater Mn(IV)/Mn(III) ratio in soil Mn oxides enhances Cr oxidation.

Accurate measurement of nitrification rates in disturbance-sensitive soils can be problematic. A series of experiments illustrated the extent of net nitrification increases and explored possible causes. In highly disturbed soils, weekly leachates of columns containing homogenized Oi/Oe material had N[O.sup.-.sub.3] concentrations over 5 mmol [L.sup.-1] after 20 wk and over 1 mmol [L.sup.-1] after 2 yr. Small-scale impact disturbance in the field (footprints) resulted in a tripling of N[O.sup.-.sub.3] concentrations after a 2-wk incubation. After disturbance by sampling, bulked Oa horizons showed rapid increases in N[O.sup.-.sub.3] concentration, with change detectable within 1 h. Initial net nitrification rates were as high as 74 [micro]mol [kg.sup.-1] [h.sup.-1] (300 mg N [m.sup.-2] [d.sup.-1]). Ammonium concentrations increased over the first 10 h but decreased afterwards. Incubation with acetylene stopped nitrate accumulation but N[H.sup.+.sub.4] continued to increase. Intact cores also showed increases in N[O.sup.-.sub.3] after 1 to 3 d but at a much lower rate than bulked samples. Gross rates of nitrification in intact cores were much lower than net rates in bulk samples. The addition of N[H.sup.+.sub.4] to intact cores caused an increase in net nitrate after 20 h. These results suggest that sampling stimulates nitrification rapidly through an increase in N[H.sup.+.sub.4] availability. The N[H.sup.+.sub.4] increase may be because of a stimulation of ammonification caused by mixing, a decrease in spatial heterogeneity, or a disturbance in other types of N[H.sup.+.sub.4] consumption. In soils that exhibit these disturbance effects, nitrification rate measurements will not be accurate but still may represent the potential for nitrification with increased N[H.sup.+.sub.4] supply.

The chemistry of extremely acid Oa horizons does not conform to traditional pH, Al, and base saturation relationships. Results from two separate studies of northeastern U.S. forested soils were used to investigate relationships between pH in water or dilute salt solutions and other soil characteristics. In Oa horizons with pH below 4, soil pH in dilute Ca[Cl.sub.2] solution was correlated with exchangeable [H.sup.+] measured either by titration (r = -0.88, P = 0.0001, n = 142) or by electrode (r = -0.89, P = 0.0001, n = 45). Exchangeable [H.sup.+] expressed as a percentage of the cation-exchange capacity (CEC) was linear with pH and showed similar slopes for data from both studies. For all samples, pHw = 4.21 - 1.80 x [H.sup.+]/CEC ([R.sup.2] = 0.69, n = 194). The reciprocal of the [H.sup.+]/CEC ratio is base saturation with Al added to the bases. Because of the low pH, exchangeable Al does not appear to behave as an acid. Exchangeable [H.sup.+] remains an operationally defined quantity because of the difficulty in separating exchange and hydrolysis reactions. In a variety of neutral-salt extractants, concentrations of [H.sup.+] were correlated with 0.1 M Ba[Cl.sub.2]-exchangeable [H.sup.+] (r > 0.91, P = 0.0001, n = 26) regardless of the strength of the extract. Nine successive extractions with 0.33 mM Ca[Cl.sub.2] removed more [H.sup.+] than was removed by single batch extractions with either 1 M KCl or 0.1 M Ba[Cl.sub.2] (average [H.sup.+] of 70, 43, and 49 mmol [kg.sup.-1], respectively for 26 samples). The data showed little difference in the chemical behavior of Oa horizons from a variety of geographical sites and vegetation types.

A test experiment involving the the relative effects of acidity and ionic strength on the chemistry of leachates from fresh, intact forest floor covers is discussed. Acidic and nonacidic treatments were conducted at each concentration. The chemical composition of field-extracted soil solutions were compared with laboratory leachates. Results indicate that the ionic strength ofincoming leaching water determined the downward movement of acidity in soils.